A plasma reactor may be used to clean oxides from a conductor or metal surface on a wafer. In operation, a wafer is placed upon a support surface or a pedestal within the process chamber. A process gas is introduced into the process chamber to perform the etching process and a plasma is formed in the process chamber proximate to the surface of the wafer. The plasma is ignited by coupling a substantial amount of energy to the process gas. The energy is typically supplied by an antenna surrounding the process chamber. The antenna is usually powered by an RF power source or a high AC or DC voltage applied between the chamber walls and the pedestal supporting the wafer.
Due to the large amount of energy which is coupled to the process gas to form the plasma, the process chamber can become quite hot. In order to control wafer temperature during the process, the wafer is clamped to a pedestal and a gas is circulated between the wafer and the pedestal. Due to the circulation of the gas, the wafer may be disturbed and move from its resting position upon the pedestal. Therefore, a clamping mechanism is needed to hold the wafer in place during the etching process.
In some instances, a mechanical lift assembly is used to hold the wafer in place. The lift assembly typically includes a cylindrical ring which is positioned around the etching surface of the wafer to hold the wafer in place upon the pedestal during the etching process. After etching is completed, the ring is lifted off the wafer. A robotic arm removes the etched wafer from the pedestal and replaces it with another wafer to be etched. To keep the procedure moving quickly, it is necessary that the cylindrical ring used for clamping be readily removable from the wafer.
One known type of cylindrical ring used to hold the wafer in place during the etching process is shown in FIG. 1. It includes a rim 4 made from an insulating material, such as Dupont Vespel.RTM., and which is attached to a lift cylinder (not shown). The lift cylinder moves in a vertical direction and is used to move the rim 4 onto and off of the wafer. The rim 4 is typically attached to the lift cylinder by metal screws. Plastic or other type screws are generally unsatisfactory as they may melt or the elasticity of the plastic does not allow for sufficient clamping force to be applied to the wafer. If the metal screws are exposed to the plasma in the reactor chamber, a reaction takes place and impurities in the metal screws are introduced into the process gas. In order to prevent the metal screw from being exposed to the process gas and corrupting the etching process, a cover 8 is placed over the rim 4. The cover 8 is also made from an insulating material, such as Dupont Vespel.RTM., which is compatible with the process chemistry and process conditions. However, the cover 8 wears out quickly, adding to the cost of operation.
Alternative designs have been proposed which eliminate the need for a mechanically fastened rim and cover. For example, U.S. Pat. No. 5,486,975 describes an electrostatic chuck which is used to hold a silicon wafer in place on the pedestal. Electrostatic chucks generally contain a pedestal supporting a dielectric layer and having an embedded electrode. An electrostatic clamping force is created between the wafer and a wafer support surface of the chuck by connecting the electrode to a high voltage, DC power supply. During the etching process, coulomb forces retain the wafer on the support surface of the chuck. U.S. Pat. Nos. 5,688,358 and 5,740,009 also show different types of electrostatic chucks.
Unfortunately, electrostatic chucks and other alternatives typically require an expensive design. Further, charge sensitive processes cannot tolerate process conditions associated with such electrostatic chuck designs. Therefore, there is a need for a cost-effective way to improve the life of the wafer lift assembly for processes which use clamps or covers in close proximity to the wafer surface.